1. Field of the Invention
The present invention relates to an image forming system such as an electrophotographic copying machine, and more particularly, it relates to a multi-color image forming system for effecting the multi-color printing. More specifically, the present invention relates to a multi-color image forming system wherein a multi-color image is formed with at least two colors during one copy cycle, by forming a first electrostatic latent image on a uniformly charged electrostatic latent image bearing member by the first exposure, and then by developing the electrostatic latent image with developer (referred to as "toner" hereinafter) as a first toner image, then by charging the image bearing member again, then by forming a second electrostatic latent image on the image bearing member by the second exposure, and then by developing the second latent image with toner having the different color from the first one as a second toner image.
2. Related Background Art
Conventionally, electrophotographic and electrostatic image forming systems have been used terminal printers of information processing equipments such as a computer, facsimile, CAD and the like. Such a printer can obtain a record image by writing an information signal on a photosensitive member as an electrostatic latent image bearing member by a laser beam, LED, LCD or the like, then by visualizing the information signal as a toner image by a developing device, then by transferring the toner image onto a transfer sheet, and then by fixing the transferred image to the transfer sheet by a fixing device. In the past, the record image was usually a mono-color image.
However, recently, since the record images have been more clear and the information could have been more easily understood, printers which can obtain a color record image having two or more colors (for example, can indicate calculated values and data values with a color different from a color of the format or can indicate a portion of an image outputted by the CAD with a color different from a color of the other image) have been requested. Such electrophotographic apparatuses which can obtain the color image having two or more colors are mainly divided into the type that an image portion is exposed (here, "image exposure" wherein the light is applied to portions corresponding to dark portions of an original image) and then is subjected to the reversal development (referred to as "image exposure/reversal development type" hereinafter) and the type that a non-image portion in exposed (background exposure) and is subjected to the normal development (referred to as "background exposure/normal development type" hereinafter).
First of all, the image exposure/reversal development type will briefly be explained with reference to FIG. 4. In FIG. 4, the reference numeral 1 denotes a drum-shaped (or belt-shaped) photosensitive member as an image bearing member rotated in a direction shown by the arrow a in FIG. 4 and having a photoconductive layer made of amorphous silicone or the like. Incidentally, the change in surface potential of the photosensitive member (photosensitive body) during the following steps is shown in FIGS. 6 to 11. First of all, the photosensitive member 1 is uniformly charged to for example+400 volts (FIG. 6) by a first charger 2, and then is exposed by the first image exposure 3.
The first exposure 3 is effected by a first laser beam emitted from a first semi-conductor laser 11 as a light source and modulated in response to a first image signal. The first laser beam is polarized by a polygonal mirror 13 rotated at a constant number of rotation by a motor 14 and then passes through a focusing lens 15 to reach a reflection mirror 16. Then, the first laser beam reflected by the reflection mirror 16 luster-scans the surface of the photosensitive member 1, so that the surface potential of the exposed portion is attenuated to for example +100 volts (FIG. 7), thereby forming a first latent image.
The first latent image is developed by a first developing device 4, for example, containing black one-component magnetic toner charged positively. A first reversal development of the first latent image is effected by applying to the first developing device 4 a bias voltage obtained for example by overlapping a DC voltage of +340 V to an AC voltage of 2000 Hz, 1300 Vpp (FIG. 8). The potential of a first toner image T1 (FIG. 8) formed in this way becomes about +200 volts since it is increased by the toner charge by about +100 volts.
After the first reversal development, the photosensitive member 1 is charged again by a second charger 5, thereby increasing the potential of the first toner image to +420 volts, for example (FIG. 9). Then, the photosensitive member 1 is exposed by the second exposure 6. The second exposure 6 is effected by a second laser beam emitted from a second semiconductor laser 12 as a light source and modulated in response to a second image signal. The second laser beam is polarized by a rotating polygonal mirror 13 and then passes through a focusing lens 15 to luster-scan the surface of the photosensitive member 1, so that the potential of the exposed portion is attenuated to for example +60 volts (FIG. 10), thereby forming a second latent image.
The second latent image is developed by a second developing device 7, for example, containing red one-component non-magnetic toner charged positively. A second reversal development of the second latent image is effected by applying to the second developing device 7 a bias voltage obtained for example by overlapping a DC voltage of +400 V to an AC voltage of 1600 Hz, 1300 Vpp (FIG. 8), thereby forming a second toner image T2 (FIG. 11).
The two different color toner images T1, T2 formed on the photosensitive member 1 in this way are transferred onto a transfer sheet P by a transfer device 8 and then are fixed to the transfer sheet by a fixing device 9. On the other hand, the residual toner remaining on the photosensitive member 1 is removed and collected by a cleaning device 10.
Next, the background/normal development type will be explained also with reference to FIG. 4. Incidentally, the change in surface potential of the photosensitive member 1 during the following steps is shown in FIGS. 12 to 17.
In this background/normal development type, after the photosensitive member 1 is uniformly charged to for example +400 volts by a first charger 2 (FIG. 12), a non-image portion is exposed by the first exposure 3. The first exposure 3 is effected by a first laser beam emitted from a first semi-conductor laser 11 as a light source and modulated in response to a first image signal. The first laser beam is polarized by a polygonal mirror 13 rotated at a constant number of rotations by a motor 14 and then passes through a focusing lens 15 to reach a reflection mirror 16. Then, the first laser beam reflected by the reflection mirror 16 luster-scans the surface of the photosensitive member 1, so that the surface potential of the exposed portion is attenuated to for example +40 volts (FIG. 13), thereby forming a first reversal latent image.
The first latent image is developed by a first developing device 4, for example, containing black one-component magnetic toner charged negatively. A first normal development of the first latent image is effected by applying to the first developing device 4 a bias voltage obtained for example by overlapping a DC voltage of +110 V to an AC voltage of 2000 Hz, 1300 Vpp (FIG. 14). The potential of a first toner image T1 (FIG. 14) formed in this way becomes about +320 volts since it is decreased by the toner charge by about +80 volts.
After the first normal development, the photosensitive member 1 is charged again by a second charger 5, thereby increased the potential of the first toner image to +420 volts, for example (FIG. 15). Then, the photosensitive member 1 is exposed by the second exposure 6. The second exposure 6 is effected by a second laser beam emitted from a second semiconductor laser 12 as a light source and modulated in response to a second image signal. The second laser beam is polarized by a rotating polygonal mirror 13 and then passes through a focusing lens 15 to luster-scan the surface of the photosensitive member 1, so that the potential of the exposed portion is attenuated to for example +40 volts (FIG. 16), thereby forming a second reversal latent image.
The second latent image is developed by a second developing device 7, for example, containing red one-component non-magnetic toner charged negatively. A second normal development of the second latent image is effected by applying to the second developing device 7 a bias voltage obtained for example by overlapping a DC voltage of +150 V to an AC voltage of 1600 Hz, 1300 Vpp (FIG. 17), thereby forming a second toner image T2.
The two different color toner images T1, T2 formed on the photosensitive member 1 in this way are simultaneously transferred onto a transfer sheet P by a transfer device 8 and then are fixed to the transfer sheet by a fixing device 9. On the other hand, the residual toner remaining on the phtosensitive member 1 is removed and collected by a cleaning device 10.
It is surely possible to obtain the two color image according to the above-mentioned exposure/development types. However, as a result that the image was obtained by the above conventional exposure/development technique, it was found that, at the peripheral zone of the copied or formed image, the color toner was adhered at areas where any images should not have exist essentially, with the result that the copied two color image was not distinct. Such indistinct image was formed not only by the reversal development but also by the normal development. Several reasons seems to be considered. For example, particularly, in case of a thicker original, when the original rested on an original support is read by a scanner to convert image information into an image signal, a shade is likely to occur at a peripheral area of the original, with the result that the shade is also converted into the image signal (particularly, although depending upon color discriminating techniques, such shade is likely to be discriminated as a color signal). If the color toner is adhered to the peripheral zone of the image, the image will be indistinct considerably.